The only answer to this currently available in the Helix is to place it between amp "amp blocK' and a "cabinet block or ir block". It may not be 100% accurate, and you may wish it could be placed between the PRE and POWER section, but that is the BEST solution at this time.
There may come a time with Helix offers a selectable power amp as an option. Single ended 6v6, push pull EL84, EL34, 6L6, etc... etc.. but you would still be playing the accuracy game because it wouldn't be the exact power amp for each model, it would just be "close to one". What PI style does it model, what output transfer gets modeled, etc... etc...
If you really want to get creative... try this.
You could likely create a static IR block to simulate the tone of the power amp. Using a DR as an example, these would be the steps....
Insert a Deluxe Reverb AMP Block and a PREAMP model of the same amp. MUTE the preamp model for now.
Insert a CAB block or IR after the amp blocks
Dial in the tone your want with the full AMP Block
Now mute that AMP Block and turn on the PREAMP block.... set the values the exact same as the Full Amp Block
Turn off the cab/ir block... it was only there so you could dial in a tone you like.
Now go through the process of creating an IR using a tone print of the PREMAP model, and a separate tone print of the Full AMP model.
Once you have the IR, you re-assemble your amp block in this manner.
PREAMP model
Inert Spring Reverb
IR of the power amp you just created
Cab or Cab IR block
The power amp will be a static/fixed tone, not dynamic like a real amp. That just means it won't change as you adjust the pre-amp, but if you captured the tone you like it will be very close!